Some timepiece balances springs, such as Breguet overcoils, include an outer terminal curve in a particular bent shape, or in a particular curve such as the Phillips curve, and the terminal curve is pinned up to a stud. In the case of a flat balance spring, this balance spring stud is in a different plane to that of the spring, and the projection of its position into the plane of the spring can be located anywhere relative thereto, within or outside the spring's range of movement. In the case of a cylindrical or other type of balance spring, the stud may occupy any position in space.
The use of silicon has allowed great progress to be made in watch making, in particular by using silicon hairsprings for high oscillation frequencies, notably 10 Hz.
Techniques implementing silicon enable flat components to be made, by DRIE (deep reactive ion etching), and complex geometries to be obtained. For three-dimensional components, the fabrication possibilities are limited to parallel, multi-layered components, and it is possible to combine various fabrication methods: assembly, multi-level etching, wafer bonding or others. These fabrication methods are generally limited to putting together flat sub-components, which may be staged, assembled in different levels.
It is not possible, with these techniques, to fabricate a bent balance spring with an outer terminal curve rising in a gentle slope upwards to the higher point of attachment to the stud. Nor is it possible to make components with strong curves.
Indeed, if the stud is located much higher than the plane of the balance spring, the terminal curve must allow a proper assembly to be made between the body of the balance spring and the stud.
Thus, in order to overcome these mechanical problems, complex silicon parts in three-dimensions need to be made.
EP Patent Application No. 2 184 652 in the name of MONTRES BREGUET proposes a paraxial solution with an assembly between two parallel flat curves in micro-machinable material via a joint bar perpendicular to the planes of these two flat curves, which constitutes substantial progress relative to the prior art.
EP Patent Application No. 2 196 867 A1 in the name of MONTRES BREGUET discloses a balance spring with terminal curve elevation, made of silicon-based material, including an elevation device between the outer coil and the terminal coil of the balance spring.
EP Patent Application No. 1 843 227 A1 in the name of The Swatch Group Research and Development Ltd discloses a coupled resonator including a balance spring and a tuning fork, resonating at different frequencies, and including permanent mechanical coupling means.